Factbox: Ties binding China and North Korea

SEOUL (Reuters) - North Korean leader Kim Jong-il is visiting China, his only major ally, accompanied by his son and likely heir apparent, sources in Seoul have said.

Here are some facts about ties between the two countries.

COMRADES-IN-ARMS

Communist China was a key backer of North Korean Communist forces in the Korean War, and sent soldiers across the border into Korea from October 1950.

After the 1953 armistice, China kept supporting North Korea, helping with its rebuilding. In 1961, the two countries signed a treaty which calls for either to aid the other if attacked. It remains in force, but its potential application is ambiguous.

After China’s rapprochement with the West and then its establishment of formal diplomatic ties with South Korea in 1992, ties between Beijing and Pyongyang frayed.

In recent years, China has sought to shore up relations and increased aid to its poor neighbor, which it sees as a strategic buffer against the U.S. and its regional allies.

In early May, North Korean leader Kim Jong-il visited China on his first trip abroad since 2006.

SHIP SINKING TENSIONS

The sinking of a South Korean navy ship on March 26 deepened tensions between Pyongyang and Seoul and hurt Chinese ties with South Korea.

South Korea lost 46 sailors when its navy ship, the Cheonan, sank. Seoul said an inquiry found there was no doubt North Korea torpedoed the ship, but Pyongyang denied it was responsible.

China stayed low-key about the dispute, reflecting its desire to stay friendly with both North and South Korea. Beijing’s approach irked Seoul, which wanted heavier pressure on Pyongyang.

In July, China backed a U.N. Security Council statement that condemned the sinking, but stopped short of blaming North Korea.

Since then, Beijing has been angered by joint U.S.-South Korea naval exercises that those two countries have said are aimed at warning North Korea.

Beijing has said those exercises could threaten its security and regional stability, especially if they take place in the Yellow Sea, between China and the Korean peninsula.

NUCLEAR DIPLOMACY

China has pressed North Korea to abandon its nuclear weapons, and the issue has produced cracks in their relationship.

China has sought to defuse confrontation by hosting six-party nuclear disarmament talks since August 2003. The now-stalled negotiations bring together North and South Korea, China, the United States, Japan and Russia.

In October 2006, North Korea held its first nuclear test explosion, defying public pleas from China. Beijing condemned the test and supported a U.N. Security Council resolution that authorized sanctions against North Korea.

After the North’s second nuclear test on May 25, 2009, Beijing backed a Security Council resolution authorizing more sanctions on Pyongyang, including a ban on its arms exports. In April 2009, North Korea said it was quitting the six-party talks and reversing nuclear “disablement” steps, unhappy with implementation of an initial disarmament agreement reached in 2007.

North Korea has been retreating from its earlier public renunciation of the talks, but there are no firm plans for their resumption. South Korea and the U.S. say resuming the talks will be impossible until the Cheonan ship sinking dispute is settled.

ECONOMIC FLOWS

China’s trade and aid are crucial to North Korea’s survival. In 2009, trade between China and North Korea was worth $2.7 billion, down 4 percent from 2008, according to Chinese customs statistics. In 2009, China’s bilateral trade with South Korea was worth $156.2 billion.

REFUGEES

China’s 1,415-km (880-mile) border with North Korea includes stretches of rivers that freeze over in winter, and in past years many North Korean refugees have crossed over, sometimes then making their way to other countries and then South Korea.

Outside groups have earlier estimated their numbers to be from tens of thousands to 300,000. Beijing worries that economic collapse or political turmoil in North Korea could unleash a surge of refugees into China.

(Sources: Reuters; International Crisis Group; Andrew Scobell, “China and North Korea: From Comrades-in-Arms to Allies at Arm’s Length”; U.S. Committee for Human Rights in North Korea; Congressional Research Service, “China-North Korea Relations”)